In camera units, either an electrical or a mechanical shutter can be used. If a mechanical shutter is used, precise synchronization is required between the global reset of the image sensor and the mechanical shutter. This global reset resets all the pixels (charge values) simultaneously, when the exposure of the image begins. Image exposure ends by closing the mechanical shutter. Typically a timing unit connected to the camera sensor controls all the imaging functions.
In digital still cameras and also in some palm computers, a mechanical shutter is used. The traditional connection of an image sensor to the host system is a parallel bus while synchronized imaging is used to connect HSYNC, VSYNC signals to the parallel bus. In addition, the global-reset command is conventionally implemented by means of two external signals.
Application US 2005/0057680 (Agan) discloses a method and device for controlling the imaging time in an image sensor. A row decoder and column controller controlled by the timing unit are connected to a CMOS cell. The difference between the start and end-state charge of the pixels is transferred to an ADC converter, from where it is transferred in a digital form to an image compressor. In this case, the image sensor is connected to a common bus (FIG. 7). All the other computer components, i.e. CPU, RAM, hard disk, I/O device, and CD-ROM drive are connected to the same bus too. This means that the connection of the image sensor contains as many as tens of signal leads, which clearly interferes with the miniaturization of the device.
Image sensors and their internal construction and control circuits are described in the manufacturers' data sheets. The Canon DIGIC II presents a CMOS sensor host module, which contains a considerable amount of image-processing functions. The image sensor is connected to this with quite a large number of leads. A 3.1 Megapixel CMOS image sensor, in which there is not only a pixel matrix, but also a timing unit controlling imaging, an ADC conversion unit, and a control register, is known under the name Micron MT9012. The circuit is equipped with I/O circuits for transmitting/receiving data, control commands, and timing data. Due to the parallel I/O of the ADC circuit, the number of signal leads becomes quite large.
Synchronization between a mechanical shutter system and an image sensor demands several I/O pins and there is often quite a long delay between the command signal and the actual exposure of the image. Generally, a considerable number of I/O pins are required between the host module and the image sensor.